drm/omap: panel-dsi-cm: support unbinding

Now, that the driver implements the common DRM panel API
the unbind no longer needs to be suppressed.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-37-tomi.valkeinen@ti.com